1. Field of the Invention
The present invention relates to a method of producing ferroelectric metal oxide crystalline particles of particle sizes on the order of nanometers (hereinafter referred to as “nanoparticles”) and, more particularly, to a method capable of producing high-purity, high-crystallinity ferroelectric metal oxide crystalline particles.
2. Description of the Related Art
Spontaneous polarization of a ferroelectric material has two thermodynamically stable states of opposite directions of polarization. The direction of polarization of the ferroelectric material can be changed by applying an external electric field to the ferroelectric material.
Full-scale practical application of nonvolatile memories utilizing the foregoing distinctive features of ferroelectric materials has been growing in recent years. Development of new recording mediums using ferroelectric materials have made progress in recent years. Those nonvolatile memories and new recording mediums realize high-density recording by inverting the polarization of minute regions of the ferroelectric material with an atomic force microscope or the like. A ferroelectric metal oxide having a layered structure (perovskite structure) of bismuth oxide as a crystal structure, such as SrBi2Ta2O9 (abbreviated to “SBT”), is the most suitable ferroelectric material for fabricating the nonvolatile memories and the new recording mediums.
There has been growing request for increasing the number of components per nonvolatile memory or recording medium, and miniaturization of nonvolatile memories and recording mediums in recent years. High-purity, high-crystallinity fine crystal grains of a ferroelectric metal oxide of particle sizes not greater than 50 nm are necessary to fabricate high-capacity memories having a high storage capacity not lower than 100 gigabit/in2.
A generally used conventional method of producing such fine crystal grains of a high-purity, high-crystallinity ferroelectric metal oxide deposits an amorphous precursor thin film on a silicon wafer by a sol-gel method or a CVD method, and subjects the precursor thin film to a heat treatment to crystallize the precursor thin film.
This conventional method needs to process the thin film of the ferroelectric metal oxide by the heat treatment, and hence has difficulty in simultaneously controlling both the size and the crystallinity of the fine crystal grains of the ferroelectric metal oxide. Although fine crystal grains on the order of nanometers can be produced when a ferroelectric metal oxide is heat-treated at a low temperature, a large amount of impurities remain in the ferroelectric metal oxide; and the crystallinity is reduced because the temperature for the heat treatment is low. The nanoparticles of the ferroelectric metal oxide thus produced are not suitable for use as a ferroelectric material for forming nonvolatile memories and new recording mediums. On the other hand, although high-purity, high-crystallinity fine crystal grains of a ferroelectric metal oxide can be produced when the ferroelectric metal oxide is heat-treated at a high temperature, crystals grow excessively and it is difficult to produce fine crystal grains on the order of nanometers.